Oven temperature control



July H, 1967 A. DEL DUCA OVEN TEMPERATURE CONTROL Filed Feb. 12, 1960 FYZ'G. 2.,

FIQB 2,

/N VE/V 7'0)? ANTHONY 05/. 0004 BY H/S A TTOEA/EYS HARE/5, K/ECH, RUSSELL (3; KERN 3,330,941 OVEN TEMPERATURE CONTROL Anthony Del Duca, Garden Grove, Calif., assignor to Beckman Instruments, Inc., a corporation of California Filed Feb. 12, 1960, Ser. No. 8,316 3 Claims. (Cl. 219-501) This invention relates to ovens for operation at substantially constant temperatures and, in particular, to an oven with a combined heat source and temperature control circuit.

Thermostats are often used in temperature control circuits but have two features which are undesirable in ject is to provide an oven adapted for use with various components the temperature of which is to be controlled and in which such components may be easily installed and replaced.

A further object is to provide such an oven that may be operated from an AC. voltage source or a DC. voltage source. a

It is an object of the invention to provide a plug-in type temperature controlled oven having a case with a plug mounted at one end and a housing at the other end, a socket carried within .the case for receiving a circuit component, and a heat source mounted in the case between the plug and socket with the heat source com- 'prisinga transistor and a voltage divider adapted for connection through the plug to a .voltage source, with the voltage divider providing a current control for the transistor and with the transistor providing the heat for the oven. v

The invention also comprises novel details of construction and novel combinations and arrangements of parts, which will more fully appear in the course of the following description. The drawing merely shows and the description merely describes preferred embodiments of the present invention which are given by way of illustration or example. i

In the drawing:

FIG. 1 is a sectional view of a preferred form of the invention;

FIG. 2 is a schematic diagram of the circuitry of FIG. 1; and

FIG. 3 is a schematic diagram of an alternative circuit.

The oven of the present invention is particularly suitable for use in operating small electrical components at a substantially constant temperature. Typical components are a vacuum tube, a crystal, a transistor oscillator, a printed circuit board, sample and reference cells in colorimetric and spectrophotometric apparatus, and the like. The oven is illustrated herein in conjunction with a sevenpin miniature type vacuum tube, but, of course, any other component may be substituted therefor.

The oven includes a case 10 having an octal type plug 11 mounted at the lower end thereof with a retaining ring 12. A housing 13 is a push fit over the upper end of the case 10, with the housing, the case and the plug enclosing the oven space. A conventional seven-pin miniature socket 14 is mounted on an annular flange 15 fixed to the interior of the case 10. The component the temperature of which is to be controlled, a vacuum tube 16,

United States Patent 3,330,941 Patented July 11, 1967 is plugged into the socket 14 and may be retained in place by a conventional tube shield 17.

A control circuit and heat source comprising a resistor 20, a resistor 21, a transistor 22 and a diode 23 is mounted within the case between the plug 11 and the socket 14. The socket 14 and the circuit component-s 20-23 are wired to the plug 11, permitting the oven to be installed and replaced at will in other electrical apparatus.

A Wiring diagram for the circuitry of FIG. 1 is shown in FIG. 2,. The components 20-23 are positioned within a chamber 26, corresponding to the oven of FIG. 1. The cathode of the diode 23 is connected to one terminal of a secondary winding 27 of a transformer 28 and the anode of the diode is connected to the collector element of the transistor 22. The emitter element of the transistor is connected to a tap on the secondary winding 27 and the base of the transistor is connected to a junction point 29 between the resistors 20, 21. These resistors function as a voltage divider and are connected in series across the ends of the secondary winding 27.

The resistor 20 is selected to have a resistance that varies with the temperature thereof, while the resistor 21 is selected to have a resistance that is substantially independent of its temperature. The power dissipated in the transistor provides the heat for the oven, the power and, hence, the heat being a function of the current therein. The transistor current is a function of the base-emitter potential which in turn is controlled by the relative resistances of the resistors 20, 21. In considering the operation of the circuit, assume that the end 30 of the winding 27 is negative with respect to the end 31 and that the temperature within the oven is higher than desired. The resistor 20 is selected to have a positive temperature coeflicient of resistance and, hence, its resistance will be greater than its normal value. Under these conditions, junction 29 will be more postive than normal and the transistor current will be reduced or completely cut otf. Then the heat from the transistor will be reduced or eliminated and the temperature of the oven will drop. As the oven temperature falls, the resistance of the resistor 20 will also reduce to increase the'heat supplied by the transistor, so as to maintain the oven at a particular operating temperature. Similary, if the temperature of the .oven drops below the desired value, the resistance of the resistor 20 also drops below its normal value and the current in the transsistor. is increased to increase the heat input to the oven. The diode 23 substantially limits current in the transistor to the half cycle when the collector is negative with respect to the emitter. Typically, the diode is a semiconductor device but any type of unidirectional rectifier may be used.

When the oven is operated as described above, an equilibrium point will be reached at the temperature where the power dissipated in the transistor is equal to the power lost from the oven by conduction, convection and radiation. The magnitude of the equilibrium temperature point can be controlled by controlling the value of the electrical power source and by the relative resistances of the resistors 20, 21.

In the circuit of FIG. 2, the resistor 20 has a positive temperature coefficient while the resistor 21 preferably has a substantially zero temperature coefiicient. The sensitivity of the control circuit can be increased by selecting the resistor 21 to have a negative temperature coefiicient. The sensitivity can also be increased by providing amplification between the junction 29 and the transistor base. As another alternative, the resistor 20 may have a substantially zero temperature coefiicient while the resistor 21 has a negative temperature coeflicient. Of course, a npn transistor can be used instead of a pnp transistor, with appropriate polarity changes in the circuit.

was operated at a five to six watt level.

An alernative form suitable for operation from a DC. source is shown in FIG. 3, wherein components identical to those of FIG. 2 are identified by the same reference numerals. The transistor collector is connected to the negative end of a DC power source 34 and the transistor emitter is connected to an intermediate voltage tap. The

voltage divider comprising the resistors 20, 21 is connected across the source 34. The operation of this circuit is the same as that of FIG. 2. In both FIGS. 2 and 3 it should be noted thatonly the transsistor and the temperature sensitive resistor need be placed within the oven, the control circuit being fully operative with the other resistor and the diode, where used, positioned exteriorly of the oven itself. 7

Although exemplary embodiments of the invention have been disclosed anddiscussed, it will be understood that other applications of the invention are possible and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim as my invention:

.1. In an oven, the combination of: chamber; a source of AC. voltage having first and second end terminals and an intermediate terminal; a transistor positioned within said chamber and having a collector element, an emitter element and a base, with said elements connected across said intermediate and first terminals of said source; a rectifier unit connected in series with said collector element for unidirectionalcurrent in said transistor; a first resistor connected across said first terminal and said transistor base; and a second resistor connected across said second terminal and said transistor base, with at least one of said resistors positioned within said chamber and with the resistance of said one resistor varying as a function of the temperature thereof for controlling the current in said transistor and, hence, the heat produced thereby.

2. In an oven, the combination of: an enclosed chamber; a source of AC. voltage having first and second end terminals and an intermediate terminal; a transistor positioned within said chamber and having a collector element, and emitter element and a base, with said elements an enclosed 4 connected across said intermediate and first terminals of said source and with power dissipation in said transistor providing the heat within said chamber; a rectifier unit connected in series with said collector element for unidirectional current in said transistor; a first resistor positioned within said chamber and connected across said first terminal and said transistor base, with the resistance of said first resistor varying as a function of the temperature thereof; and a second resistor positioned within said chamber and connected across said second terminal and said transistor base, with the resistance of said second resistor substantially independent of the temperature thereof.

3. In a plug-in type temperature controlled oven, the

combination of: a case; a plug mounted at one end of said case; a housing mounted at the other end of said case, with said housing, case and plug defining an oven space; a socket carried within said case for receiving a circuit component disposed within said oven space; and a heat source mounted in said case between said plug and socket,

.voltage source, with a tap of said divider connected to said transistor base, and with the potential at said tap varying as a function of the temperature of said divider for controlling the current in said transistor and, hence, the heat produced thereby.

References Cited UNITED STATES PATENTS 2,846,592 8/1958 Rutz 250-211 2,859,402 11/ 1959 Schaeve. 2,932,714 4/ 1960 Merrill. 2,938,130 5/1960 Noll. 2,975,260 3/ 1961 Carlson. 2,984,729 5/1961 Hykes et al. 2,994,759 8/1961 Lipman. 3,028,473 4/1962 Dyer et al. 219-2041 3,079,484 2/ 1963 Shockley .et al. 219-501 3,300,623 1/ 1967 Smyrnos 219-501 OTHER REFERENCES Horne et al.: Review of Scientific Instruments; vol. 30, No. 12; December 1959; pp. 1132-1134.

RICHARD M. WOOD, Primary Examiner.

W. STOLWEIN, Examiner. L, H, BENDER, Assistant Examiner, 

1. IN AN OVEN, THE COMBINATION OF: AN ENCLOSED CHAMBER; A SOURCE OF A.C. VOLTAGE HAVING FIRST AND SECOND END TERMINALS AND AN INTERMEDIATE TERMINAL; A TRANSISTOR POSITIONED WITHIN SAID CHAMBER AND HAVING A COLLECTOR ELEMENT, AN EMITTER ELEMENT AND A BASE, WITH SAID ELEMENTS CONNECTED ACROSS SAID INTERMEDIATE AND FIRST TERMINALS OF SAID SOURCE; A RECTIFIER UNIT CONNECTED IN SERIES WITH SAID COLLECTOR ELEMENT FOR UNIDIRECTIONAL CURRENT IN SAID TRANSISTOR; A FIRST RESISTOR CONNECTED ACROSS SAID FIRST TERMINAL AND SAID TRANSISTOR BASE; AND A SECOND RESISTOR CONNECTED ACROSS SAID SECOND TERMINAL AND SAID 